1. Field of the Invention
The present invention is directed to a discharge lamp driving circuit, and more particularly to a circuit for operating a gaseous discharge lamp utilizing a bridge inverter having a relatively low switching frequency.
2. Description of the Prior Art
There has been a growing demand for discharge lamp operating circuits which are operated at a higher frequency in order to reduce the weight and bulk of the ballasting inductor. On the other hand, it is known that discharge lamps, particularly high-pressure discharge lamps such as mercury high pressure lamps and sodium vapor lamps suffer from unstable discharge arcs due to "acoustic resonance" when operated at certain high frequencies which will vary in different lamps but normally lies within the high frequency range between 10 KHz to 100 KHz. Thus, the high pressure discharge lamp is required to be operated at a frequency low enough with respect to the high frequency in which the acoustic resonance is expected. One known scheme for satisfying the above two conflicting requirements is shown in U.S. Pat. No. 4,170,747 which utilizes a bridge inverter including two pairs of switching elements or transistors for operating the discharge lamp connected in series with the ballasting or current limiting inductor across the output terminals of the bridge inverter. One pair of the switching transistors operates at a lower frequency for alternately applying a dc voltage in opposite polarity to the lamp for the purpose of avoiding the acoustic resonance, while the other pair of the switching transistors operates to repetitively interrupt the dc voltage being applied to the lamp at a higher frequency in order to reduce the bulk and weight of the current-limiting inductor involved to a considerable extent. The high frequency component is bypassed through a capacitor connected across the lamp and will not induce the acoustic resonance. In view of the low switching frequency at which the bridge inverter provides the alternating voltage to the lamp, this patent also envisages the prevention of short circuiting of the power source by providing an all-off period during which all of the transistors are off or nonconducting. In other words, the transistors of the bridge inverter would be possibly damaged due to the short-circuiting of the power source without the provision of the all-off period. At the initial stage of the all-off period, the inductor and the capacitor connected to the lamp act to continuously cause the lagged current to flow to the lamp to maintain the lamp in the conductive state. However, this lamp current with progressively decreased amplitude flows only in one direction and therefore will be reduced to zero only in a short time. When the lamp current completely ceases within the all-off period, the lamp requires a higher reignition voltage at the subsequent conduction of the switching transistor of the bridge inverter, which higher reignition voltage could disadvantageously lead to extinction, or at least the flickering of the lamp.
This poses a problem that the all-off period is substantially limited to a reduced duration which may not be safe enough for preventing the short-circuiting of the power source in consideration of the inevitable characteristic variations of the electric components forming the circuit. In other words, the extinction or flicker problem will be critical when the circuit is designed to provide the all-off period of enough duration for prevention of the short-circuiting of the power source. In this sense, the prior art circuit is not satisfactory for stable lamp operation.